Different substances exhibit different absorption properties with reference to X-radiation as a function of the energy of the X-radiation. On the basis of the different absorption properties, an image in which substantially only a single substance still remains visible can be calculated by a weighted subtraction of a high energy X-ray image from a low energy X-ray image.
Such a method is known, for example, from FujiFilm Co, Technical review no. 12, “Upright image reader that supports energy subtraction processing software”. The low energy X-ray image and the high energy X-ray image are acquired by means of an energy-selective detector and calculated with one another to form a result image in which essentially either bone tissue or soft tissue is present. The calculation of the result image is performed at the level of individual pixels, the attenuation values of respectively corresponding pixels of the two X-ray images being subtracted from one another while taking account of substance-specific weights.
The substance-specific weighting takes account of the fact that in the case of two different energies of the X-radiation a defined pair of attenuation values is produced for the substance under ideal boundary conditions in relation to each pixel of the result image. If the two attenuation values relating to the different energies of the X-radiation are regarded as a pair of measured values of a two-dimensional feature space, the pairs of measured values of a substance are thus respectively imaged at the same point in the feature space.
The calculation, referred to individual pixels, of a result image, for example a segmented image, leads, however, in many situations to a defective result. A substantial reason is to be seen in that the segmentation criterion with the aid of which the presence of the substance is decided is based on the assumption that the two attenuation values are produced on the basis of a defined composition of the substance. However, in many situations the actually existing composition of the substance deviates substantially from the ideally assumed composition. Moreover, the attenuation values are falsified by measurement noise. Pairs of measured values of the substance are then not imaged onto one point in the feature space.
Under real examination conditions, different substances can be imaged onto zones in the feature space that exhibits an intersection set. Thus, for example, pairs of measured values of the two different substances of bone and iodine solution that are produced in conjunction with a set voltage of 80 and 140 kV are imaged onto zones with an intersection set in the range between approximately 100 HU (Hounsfield Units) and 200 HU. A unique assignment of these pairs of measured values to a substance is not possible in principle on the basis of an evaluation only of pixels corresponding to the two attenuation values.